Triple Oxygen Isotope Measurements (Δ'17O) of Body Water Reflect Water Intake, Metabolism, and δ18O of Ingested Water in Passerines

Abstract

Understanding physiological traits and ecological conditions that influence a species reliance on metabolic water is critical to creating accurate physiological models that can assess their ability to adapt to environmental perturbations (e.g., drought) that impact water availability. However, relatively few studies have examined variation in the sources of water animals use to maintain water balance, and even fewer have focused on the role of metabolic water. A key reason is methodological limitations. Here, we applied a new method that measures the triple oxygen isotopic composition of a single blood sample to estimate the contribution of metabolic water to the body water pool of three passerine species. This approach relies on Δ'17O, defined as the residual from the tight linear correlation that naturally exists between δ17O and δ18O values. Importantly, Δ'17O is relatively insensitive to key fractionation processes, such as Rayleigh distillation in the water cycle that have hindered previous isotope-based assessments of animal water balance. We evaluated the effects of changes in metabolic rate and water intake on Δ'17O values of captive rufous-collared sparrows (Zonotrichia capensis) and two invertivorous passerine species in the genus Cinclodes from the field. As predicted, colder acclimation temperatures induced increases in metabolic rate, decreases in water intake, and increases in the contribution of metabolic water to the body water pool of Z. capensis, causing a consistent change in Δ'17O. Measurement of Δ'17O also provides an estimate of the δ18O composition of ingested pre-formed (drinking/food) water. Estimated δ18O values of drinking/food water for captive Z. capensis were ~ −11‰, which is consistent with that of tap water in Santiago, Chile. In contrast, δ18O values of drinking/food water ingested by wild-caught Cinclodes were similar to that of seawater, which is consistent with their reliance on marine resources. Our results confirm the utility of this method for quantifying the relative contribution of metabolic versus pre-formed drinking/food water to the body water pool in birds.

A multi-isotope approach reveals seasonal variation in the reliance on marine resources, production of metabolic water, and ingestion of seawater by two species of coastal passerine to maintain water balance

“…Triple oxygen analysis: Water budget and water sources Δ 17 O results suggest that the contribution of metabolic water to the total water budget in C. nigrofumosus was slightly higher in summer (~25%) than in winter (~21%), in agreement with expectations based on differences in BMR between seasons. Both estimates are slightly lower to previously reported 17 O-based estimates for C. nigrofumosus (~28%) sampled from another locality ~200 km to the north of our field site (Sabat et al, 2021). For C. oustaleti, estimates of the metabolic water contribution (23%) were nearly identical to those reported for this species from the more northern locality (Sabat et al, 2021).…”

“…Output from the H 2 O (kPa) analyzer, the oxygen analyzer (%), and the flow meter were digitalized using a Universal Interface II (Sable Systems, Nevada, United States) and recorded on a personal computer using EXPEDATA data acquisition software (Sable Systems, Nevada, United States). To estimate BMR and TEWL, we averaged O 2 concentrations and water vapor pressures of the excurrent air stream over a 20 min period after steady state was reached, which occurs after 3 h in Cinclodes (Sabat et al, 2021). We estimated the metabolic water production (MWP) using the equivalence of 0.567 mL H 2 O per liter O 2 consumed (Schmidt-Nielsen, 1997) and calculated the ratio between metabolic water production and water losses (MWP/ TEWL) for the 20 min period during which steady state was reached.…”

“…Here, we ignore this latter contribution and acknowledge that this induces uncertainty (Whiteman et al, 2019). Of the two other sources, metabolic water is assumed to have a Δ 17 O value of −0.44‰ reflecting that of inhaled atmospheric oxygen (Whiteman et al, 2019;Wostbrock et al, 2020) (Whiteman et al, 2019;Sabat et al, 2021). We cryogenically distilled body water from blood plasma on a vacuum line.…”

“…We used carbon (δ 13 C) and nitrogen (δ 15 N) isotope analysis of feathers and blood collected during the summer and winter to characterize seasonal marine versus terrestrial resource use (Newsome et al, 2007;Martínez del Rio et al, 2009). We then used a novel methodological approach based on the measurements of the three stable isotopes of oxygen in blood plasma to measure the proportion of the body water pool that was derived from metabolic water (Whiteman et al, 2019;Passey and Levin, 2021;Sabat et al, 2021). This method utilizes natural differences in the oxygen isotope composition of preformed water versus atmospheric oxygen, which is the source of oxygen for the formation of metabolic water in the mitochondria (Whiteman et al, 2019).…”

Integrative Physiological Responses to Acute Dehydration in the Rufous-Collared Sparrow: Metabolic, Enzymatic, and Oxidative Traits

“…Maintaining osmotic balance is a challenge when birds are feeding on salty food or living in arid habitats that contain scarce freshwater (Sabat and Martinez del Rio, 2002;Sabat et al, 2009;Smit and McKechnie, 2015). Avian water balance is a function of the interplay between water availability in the environment, the physiological and behavioral mechanisms birds use to reduce water losses, and the production of metabolic water that is directly linked to metabolic rate (Gerson and Guglielmo, 2011;Albright et al, 2017;Sabat et al, 2021). Some bird species respond to environmental conditions that stress osmoregulation by increasing their rates of energy expenditure, including basal metabolic rate (BMR).…”